The present invention relates to a weighing system for a motorized hospital bed that has a drive system including motors and motor controls that are used to adjust the position of the bed. More particularly, the present invention relates to the coupling of the bed""s drive system and the bed""s weighing system so that the weighing system stores a last calculated hold weight value before any one of the bed""s motors can begin operating and then continues calculating changes in measured weight in relation to the hold value after the bed""s motors have stopped operating.
Hospital beds that use weighing systems are known in the prior art. Generally, such weighing systems include transducers that produce signals in response to the magnitude of the loads transferred through the members supporting a bed. One common approach known within the art is to use load cells or force measurement type transducers. These load cells can, for example, be formed from a unitary block of metal machined into a parallelogram configuration to provide a pair of parallel flexible members known as flexures. Strain gauges secured to the flexures produce electrical signals responsive to the magnitude of the load applied to the flexures. The strain gages of a load cell are normally connected into a wheatstone bridge type circuit that can produce a voltage change in response to a change in the resistance of the strain gages. These voltage changes can be calibrated to known loads so that load cell signals can be displayed as weight values on a standard readout device known as a weight meter.
In some therapeutic situations, it is critical that the patient""s weight be continuously tracked with a high level of precision. Beds that employ force transducer type weighing systems have a common disadvantage of measuring false changes in weight values when the bed position is changed, that is, when one section of the bed is moved relative to another section of the bed. Typically, patient weight will not change abruptly, however, the relatively abrupt movements that occur when the position of a bed is changed will cause sudden changes in the loads measured by the bed""s transducers. This causes false indications that the weight of a patient has changed. Such erroneous indications of a weight change can interfere with patient care.
Many present bed weight measurement systems provide a xe2x80x9choldxe2x80x9d function that causes a xe2x80x9choldxe2x80x9d weight value to be stored while the bed is repositioned. After a bed having a hold function is repositioned, the hold function is manually released and the weight measurement system uses the hold weight value as a new baseline by resetting the value resulting from the next set of load cell signals after release of the hold function as equal to the hold weight value. From that point, the weight measurement system calculates changes in the patient""s weight as a change in relation to the hold weight value and then as a change in the next weight value to continue producing weight values in an ongoing sequence. A manually operated hold function, however, must be activated every time a bed is repositioned. Present beds that have weighing systems and motorized systems for changing bed position do not have a means for automatically holding a weight value while a bed is repositioned. Since hospital personnel are often busy and distracted and since patients are often able to reposition their own beds, beds are often repositioned without activating a hold function. Consequently, false indications of changes in weight often occur when there is a change in bed position. What is needed is a bed having a position control system and a weight measurement system that are coupled so that the weight measurement system automatically enters into a hold mode while the position control system is changing the bed""s position.
The system of the present invention satisfies this need because it has a position control system that is coupled with the weight measurement system via a logic control unit. The bed has a logic control unit that monitors and regulates the interaction between the weight measurement system and the position control system. The position control system includes motor units that are controlled by a position control unit. The weight measurement system includes load cells and a weight meter. The load cells are situated in the load paths of the members that support the bed and include strain gauges arranged in a wheatstone bridge circuit for producing signals for the weight meter. The weight meter converts strain gauge signals from the load cells to a weight value corresponding to the load supported by the bed.
In the system of the present invention, the weight measurement system is coupled to the position control unit via a logic control unit that monitors the position control unit and activates the weight meter to store a weight measurement. When a signal is given to the logic control unit to reposition the bed, the logic control unit will first activate the weight meter to initiate a hold function wherein the weight meter stores the most current weight measurement. Once the initiation of the weight meter hold function has been verified by a feedback signal to the logic control unit, the logic control unit will activate the position control unit to reposition the bed as desired. When the position control unit has completed repositioning the bed, a second feedback signal confirms to the logic control unit the deactivation of the position control unit. The logic control unit, upon receiving this second feedback signal, sends a signal to the weight meter to deactivate the hold function and restore the meter to display the last stored weight measurement. The weight measurement system then interprets subsequent sets of load cell signals as only producing changes in relation to the last stored weight value.